In recent years, demand for power MOS semiconductors has been increasing. It thus has become increasingly important to develop N-type silicon single crystal substrates that have low resistivity and are highly doped with a volatile dopant such as antimony (Sb), arsenic (As), or phosphorus (P) as substrates for use in these semiconductors. Such silicon single crystal substrates can be obtained by slicing a silicon single crystal ingot manufactured mostly by the CZ method.
The manufacture of a silicon single crystal ingot by the CZ method begins with charging of raw material such as polycrystalline silicon to a quartz crucible disposed in a chamber. After the dopant is added to the raw material, the resultant raw material is heated and melted into a melt by a heater. A seed crystal attached to a seed holder is lowered from the upper part of the chamber to bring the seed crystal into contact with the melt. The seed crystal is rotated and gradually pulled to grow a single crystal.
As shown in FIG. 3, the growth of the single crystal involves enlarging the diameter of the single crystal to a desired diameter to form a cone, and then controlling the pulling rate and the temperature of the melt to form a straight body.
It is however very difficult for the CZ method to manufacture a single crystal highly doped with the dopant, for dislocations are often generated particularly during the period from the formation of the cone to just before the formation of the straight body. This makes the production efficiency worse. In the worst case, it may be impossible to obtain a single crystal.
In view of this problem, it is known that a taper angle of the cone is increased at least in two stages during its formation in order to inhibit the generation of the dislocations (See Patent Document 1).